Classes and Structures

Question

I have the books several actually but am currently learning VB.net 2003 with Beginning VB.net 2nd Edition by Wrox. I find it quite good although a few real life examples would be useful. I am having difficulty getting my head round Classes and Structures. Using properties public and private and where to put them. Although the book puts it forward well I am still struggling with the concept. Does anyone know where I can read an article to help me along with this? Perhaps I need to read something on the concept of OOPS programming languages? For the moment I am a new and frustrated programmer getting very bogged down. Perhaps there is someone who has gone through the same problem as myself and has been able to see the woods for the trees - unlike me for the moment. Many thanks for any help.

jones.clive · Answer

Hi MythicalMe Your efforts in trying to clear things up for me are much appreciated and I will most certainly be investigating the book you mentioned. Many thanks

MythicalMe · Answer

The difference between a class and a structure is that the structure is a value type rather than a reference type. When you define the object for a structure it is immediately initialized to it's default parameter when data is copied the actual data is copied and the equality operator isn't supported. THink of the differences in terms of usage. For instance when you define a variable that's passed to a sub you can specify either ByVal or ByRef. If you specify ByVal the actual data is passed to the sub. If you specify ByRef only a pointer is passed. Public Private Friend and Protected are known as scopes. Classes are hierchial so you need to define who can use the variable methods objects etc. The two easiest to understand are Public and Private. The private scope means that only the current class can use the structures and methods. Public is the opposite. For instance: class Parent private varA as integer public varB as integer public objC as new Child end class class Child private varA as integer public varB as string end class In this instance Parent can see and use Child's varB string but can't see or use varA. Classes can also be defined with a scope. Say I define a class within class Child as private then that class can only be used by Child. Now things start to get a little more complicated. The use of the friend scope means that anything with the assembly can be use the definition. You see this a lot with controls which are really classes. Because a control is defined as a friend in the Forms class any other class also defined in the class can use it. Classes that are outside the scope of the Form class can't use the control. Finally the protected scope is used in inheritance. If a variable (class object etc) is protected only those classes which inherit from that class can use the variable. In the previous example if varB was protected rather than public parent couldn't use varB. To use the varB the parent class has to inherit from class Child for instance: class Parent inherits Child private varA as integer public varB as integer end class class Child private varA as integer public varC as string protected varD as double end class If another class defines an object as type Parent then the scope of that object would include varB and varC. Parent's scope is varA(it's own) varB varC and varD. Note that I avoided redefining varB in the child class. If I had defined a varB then the parent class would have had to override varB to avoid a confict and compiler error. I hope that I've cleared up some of your issues. Total implementation of OOP in VB is a new concept in .NET and i've found that Francesco Balena's book an excellent source.